1. Field of the Invention
The present invention relates to a planetary gear drvice, and more specifically, to a planetary gear device having a tooth profile effective to reduce vibration and noise.
2. Description of the Prior Art
In a power transmission gear, since the gear is composed of an elastic material, it is impossible to avoid the deflection of the teeth thereof when the gear is operated with a load. Intrinsically, however, if an involute gear does not maintain a proper involute curve when it transmits rotary motion, it cannot smoothly transmit the rotary motion. The rotation of the gear is made irregular due to the deflection of the teeth and thus vibration and noise are produced and the life of the gear is shortened. Then, as proposed in, for example, Japanese Patent Unexamined Publication No. 63-180766, the deflection of the teeth of a gear operated with a load is presupposed and previously corrected so that rotary motion can be transmitted smoothly in the operation with the load.
FIG. 4 shows a sun gear and a planet gear of a planetary gear device used as a speed reduces. In this case the teeth of the gears are not deflected when no load is applied to the gears. When rotary motion is transmitted from the sun gear 1 to the planet gear 2 in the direction of an arrow, the former gear meshes with the latter gear at point a and the former is about to mesh with the latter at a point b. When no load is applied, the sun gear 1 and the planet gear 2 have the same normal pitch Pn at an adjacent tooth when measured vertically to the tooth face of the gears. The normal pitch Pn is not changed by the rotation of the gears when they are smoothly rotated. When a load is applied as shown in FIG. 5, however, the sun gear 1 meshes with the planet gear 2 at a point a in a state with great deflection (a dotted line shows the state with no deflection). On the other hand, in the meshed state at point a, the planet gear 2 has less deflection because the point a is near the dedendum of the tooth. When it is assumed that the tooth of the planet gear 2 is not deflected for the purpose of simplification, the tooth face of the tooth next to the meshed tooth of the sun gear 1 is spaced apart from the meshed tooth thereof by a distance Pn and the tooth face of the tooth next to the meshed tooth of the planet gear 2 is also spaced apart from the meshed point by a distance Pn. As a result, the next teeth collide against each other at the point b, by which vibration and noise are caused.
FIG. 6 illustrates the teeth further meshing with each other. Although the teeth are completely meshing together at point b, they are just ending their meshing at point a. Since the teeth are also meshing at point a just before this state, the load is divided into the two points and half the entire load is applied to the respective points. At the moment when the mesh of the teeth at point a is ended, however, the load is applied only to the point b with the tooth of the planet gear 2 having a loaded point far from the dedendum which causes great deflection and deformation from a state without deflection (shown by a dotted line). As a result, the teeth collide with each other at point a where the meshing ends which causes vibration and noise.
In FIG. 7 planet gear 2 meshes with an internal gear 5 in a slightly different manner than in FIGS. 4-6. Although the teeth of the planet gear 2 are completely meshing with the internal gear 5 at point a, they are about to be meshed together at a point b. Although point a is located at a high position from the dedendum of the internal gear 5, the dedendum has a relatively large tooth thickness which causes the internal gear 5 to be less deflected. On the other hand, since the point a is located at a position which is not as high from the dedendum of the planet gear 2, the planet gear 2 is also less deflected. Therefore, the deflection of the teeth of the planet gear 2 and internal gear 5 are considerably smaller than when the sun gear 1 meshes with the planet gear 2 as illustrated in FIGS. 4-6.
In a planetary gear device, two kinds of meshing occur at the same time. More specifically, they are the meshing of the sun gear 1 with the planet gear 2 and the meshing of the planet gear 2 with the internal gear 5. As described above, the teeth are deflected differently by the two kinds of meshes.
To smoothly transmit rotary motion in a loaded operation, a tooth profile must be corrected to an optimum condition by taking into consideration the deflection of teeth caused by the two kinds of meshes. Nevertheless, only the tooth profile of one of the sun gear and the planet gear is conventionally corrected by only taking into consideration the deflection caused by the meshing of the sun gear with the planet gear. Moreover, the same amount of correction is effected to the opposite faces of teeth.
In the aforesaid prior art, the deflection of teeth caused by the meshing of the planet gear with the internal gear is ignored because it is considerably smaller than the deflection of teeth caused by the meshing of the sun gear with the planet gear. At present, however, the effect of the latter deflection cannot be ignored because the reduction of vibration and noise is strongly required.